Most common fluid-circulating refrigeration systems use a vapor-compression (V-C) cycle. In this type of device, a fluid is first compressed by adding work in a substantially adiabatic process, thereby raising its pressure and temperature. The fluid then passes through a heat exchanger (condenser) where its temperature is lowered and its pressure is only minimally decreased. Next, the fluid passes through an expansion valve where it is expanded without work recovery in a substantially isenthalpic process with a subsequent decrease in pressure and temperature. This low pressure, low temperature fluid then passes through another heat exchanger (evaporator) where the fluid acts as a refrigerant and absorbs heat energy. The fluid is then returned to the compressor completing the cycle.
In the early days of mechanical refrigeration, ammonia was a common working fluid in vapor-compression systems. It is still in wide commercial use in Europe despite its toxicity and corrosive nature. Other fluids once widely used include carbon dioxide (relatively safe, but requiring high pressures), sulfur dioxide (very corrosive, strong irritant) and methyl chloride (flammable carcinogen). All these were supplanted except in specialty use by the family of chloroflourocarbons (CFC's) developed originally by the Dupont Co., many of which are non-toxic and inflammable. These compounds (for example--FREON (TM)) have come to dominate the refrigeration and air conditioning industry.
It is known that the chlorine in these CFC compounds reacts with and destroys ultraviolet radiation-absorbing ozone in the upper atmosphere. Therefore, the release of CFC's after use (or by leakage) might result in grave ecological damage. Such ozone depletion has been found and measured over both poles of the Earth, leading to an international agreement to limit the production and use of CFC's. Currently, a major effort is underway to replace these compounds in refrigeration and air-conditioning systems.
One proposed solution is to use a gas (air) cycle refrigeration system. An example of this type of system is the reverse Brayton cycle commonly used aboard jet aircraft for cooling of the cabin. Air is bled from the discharge of the main-engine compressor and then is normally expanded through a small turbine. The air becomes cool during the expansion process and is subsequently used for cooling the cabin. Thus, no liquid-gas phase change occurs in a gas cycle refrigeration system.
There are other fundamental differences between V-C and gas cycle refrigeration systems. A vapor-compression cycle has a high coefficient of performance (COP) due to its close approach to the ideal Carnot cycle. This occurs because there is nearly isothermal heat exchange in the evaporation and condensation portions of the cycle. It is clear that alternative cycles proposed to match or exceed the performance of a V-C system should also approach these ideal isothermal processes.
One other advantage of V-C machines relative to gas-cycle machines is that the large enthalpy of phase change during the isothermal processes allows for a large refrigeration effect for each mass unit of fluid. This reduces the mass of fluid required and thus the size of the attendant machinery. The inherent disadvantage of this aspect of V-C machines is that the saturation conditions of the working fluid define the operating pressure required for the refrigeration system at any desired operating temperature. It is this characteristic that has led to the almost universal use of certain CFC's (e.g. R-12) in refrigeration systems. Therefore, attention is being focused on pressurized gas cycles that can provide the same advantages as V-C machines without a dependence on a CFC as the working fluid.
Gas cycles of interest include the Brayton, Stirling and Ericsson. Comparative temperature/entropy diagrams are shown in FIG. 1.
The Brayton cycle, as already discussed (reverse cycle for refrigeration), is the most familiar. This cycle is composed of a substantially adiabatic compressor, which compresses and sends gas to a first heat exchanger where heat energy is added at substantially constant pressure (heat energy is removed in the reverse cycle). Next, the gas passes to a turbine, where it is expanded in a substantially adiabatic process. After leaving the turbine, the gas (now cooler) is directed to a second heat exchanger, where heat energy is removed at substantially constant pressure (heat energy is added in the reverse cycle). The gas then returns to the compressor thereby completing the cycle. In the idealized Brayton cycle, the compression and expansion of the gas is considered isentropic (adiabatic). The efficiency of the Brayton cycle is dependent on the pressure ratios across the compressor and turbine. The primary disadvantage of this cycle for refrigeration uses results from its constant entropy processes which require a greater net work input (therefore lower COP) for compression and expansion at given temperatures, than cycles with isothermal processes.
The Stirling cycle consists of an isothermal compression, followed by a constant volume heating. After the heating, there is an isothermal expansion stage, finally followed by a constant volume cooling. A regenerator may be placed in the system so that some of the heat rejected in the cooling stage can be used in the heating stage. This cycle offers the potential of higher performance by its isothermal (together with constant-volume) processes. Unfortunately, known practical embodiments of this cycle require discrete pistons to approximate the constant-volume processes. Stirling cycles using reciprocating hardware are currently being developed for automotive and small electricity generating systems and have even been considered for powering artificial hearts. These systems typically use a gas in a closed cycle. Since the cycle pressure is essentially uniform throughout the machine at any one time, the pressure ratio, and thereby the specific power, depend directly on the ratio of total cycle volume to cyclically-changing (swept) volume. This places severe restrictions on the sizing and placement of heat exchangers filled with cycle gas. Furthermore, such machines cannot achieve isothermal processes with pistons-in-cylinders construction; therefore all known Stirling engines are, in fact, pseudo-Stirlings. Present Stirling engines are characterized by near-adiabatic compression and expansion processes that blend without sharp distinction into the approximately constant volume legs of the cycle. For this reason, Stirling machines do not achieve high COP at the moderate temperature ratios typical of refrigeration.
The most attractive gas cycle, theoretically, is the Ericsson cycle, defined by isothermal heat exchange processes and constant-pressure temperature change processes (typically regenerated). The earliest engines designed to operate on the Ericsson cycle were essentially open-cycle versions of the Stirling cycle where valves periodically connected the cycle spaces to atmospheric pressure. These early machines, because of their low speed and low power density, attracted no more than limited interest. However, the isothermal legs of the cycle promise a high COP.
A Brayton cycle machine, with the addition of a regenerator and numerous reheat stages (associated with multi-stage turbines) and numerous intercooling stages (associated with multi-stage compressors) can approach the theoretical processes and COP of the Ericsson cycle. The major problem with this type of modification is the non-isothermal compression and expansion of each stage, requiring many stages with attendant cost and complexity to achieve high COP.